Reefer Madness has been debunked yet again, as a large study conducted in the United Kingdom failed to find any evidence of a link between cannabis use and lower intelligence among teenagers.

The study was published online January 6 in the Journal of Psychopharmacology, and it found that any statistical association between marijuana use and decreased intellectual performance vanished when researchers took other variables into account, reports Eric W. Dolan at The Raw Story.

The study looked at 2,235 teenagers who participated in the Avon Longitudinal Study of Parents and Children (ALSPAC), a longterm study following children born in the Bristol area in 1991 and 1992. The kids had their IQ tested at the age of 8, and again at the age of 15. Nearly a quarter of the participants reported having tried marijuana at least once, and 3.3 percent had used it at least 50 times.

"The notion that cannabis use itself is causally related to lower IQ and poorer educational performance was not supported in this large teenager sample," werote lead resercher Claire Mokrysz of the University College London and her colleagues.

The teenage cannabis users had lower IQ scores and worse educational performance, but the researchers found no evidence marijuana itself was to blame for the correlation. Those who used cannabis as teenagers were also more likely to have used other drugs, such as tobacco and alcohol.

“After adjustment to account for these group differences, cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance,” the researchers explained. “These findings therefore suggest that cannabis use at the modest levels used by this sample of teenagers is not by itself causally related to cognitive impairment. Instead, our findings imply that previously reported associations between adolescent cannabis use and poorer intellectual and educational outcomes may be confounded to a significant degree by related factors.”

Teenagers who had used cannabis more than 50 times had an IQ score that was only 0.1 point lower than those who had never used marijuana, on average. The findings contradict those of a smaller study from 2012 which found cannabis users could lose an average of six IQ points between ages 13 and 38. That study, published in the journal PNAS, was based on a longitudinal study of 1,037 New Zealand residents.

“Our findings suggest cannabis may not have a detrimental effect on cognition, once we account for other related factors,” Mokrysz said. “This may suggest that previous research findings showing poorer cognitive performance in cannabis users may have resulted from the lifestyle, behavior and personal history typically associated with cannabis use, rather than cannabis use itself.”

“People often believe that using cannabis can be very damaging to intellectual ability in the long-term, but it is extremely difficult to separate the direct effects of cannabis from other potential explanations,” Mokrysz said. “Adolescent cannabis use often goes hand in hand with other drug use, such as alcohol and cigarette smoking, as well as other risky lifestyle choices.

“It’s hard to know what causes what -- do kids do badly at school because they are smoking weed, or do they smoke weed because they’re doing badly?" Mokrysz said. “This study suggests it is not as simple as saying cannabis is the problem.”